One method to determine valve degradation in an internal combustion engine is described in U.S. Pat. No. 6,499,470. This method presents a method for determine degradation of a cylinder halting mechanism based on the rate of change in intake manifold pressure. If the manifold pressure rate of change is low, fuel flow can be stopped to the one or more cylinders.
The above-mentioned method can also have several disadvantages. Specifically, the method does not appear to be capable of determining a condition where operation of an exhaust valve degrades while intake valves are deactivated. For example, if intake and exhaust valves are commanded to closed positions to deactivate a cylinder, intake manifold pressure will not be affected by exhaust valve degradation because the intake valve isolates the intake manifold from conditions in the cylinder and exhaust system. As a result, the rate of change of intake manifold pressure will not be affected by operation of the exhaust valve. Consequently, the method does not appear to be able to diagnose exhaust valve degradation from at least an attribute of said spark event. This method overcomes at least some of the disadvantages of the prior art.
Valve degradation can be assessed by determining the duration of an ignition spark in a cylinder. Spark duration (i.e., the length of time from the beginning of a spark to the end of a spark) can be related to cylinder pressure and cylinder pressure can be related to the position of a valve at a particular crankshaft angle. Alternatively, breakdown voltage may be used as an indication of cylinder pressure. Consequently, spark duration and/or breakdown voltage can be used to determine the position and/or operation of a valve at a particular crankshaft angle, thereby allowing the determination of valve degradation.
During cylinder deactivation, it is common to deactivate a cylinder by closing all valves operating in a cylinder. The valves are closed to reduce engine pumping losses and to limit oxygen flow to the exhaust system after treatment devices. Exhaust gases can be trapped in deactivated cylinders to act as a spring, storing and releasing rotational energy in the cylinder. However, if an exhaust valve stays open or opens and closes during a cycle when valve deactivation is desirable, the benefit of cylinder deactivation can be reduced. The present method permits a spark to be initiated in a cylinder during a scheduled cylinder deactivation period to determine if the cylinder pressure indicates that an exhaust valve is in a desired position at a particular crankshaft interval. This allows determination of exhaust valve degradation when intake valves are deactivated in a closed position.
Further, the method can be used to determine if an exhaust valve is in a desired position during an exhaust stroke so that exhaust valve degradation can be assessed before fuel is injected to a cylinder during cylinder reactivation. For example, a spark event can be initiated in a cylinder during an exhaust stroke while exhaust valves are scheduled to open. If the spark duration indicates that cylinder pressure is higher than expected, exhaust valve degradation may be determined. If the spark duration indicates that cylinder pressure is near an expected value, then it may be determined that the exhaust valve is not degraded. The exhaust valve evaluation may then be used to determine whether or not it is desirable to inject fuel to the cylinder.
The present description may provide several advantages. In particular, the approach may enable rapid and reliable evaluation of both intake and exhaust valves. By ingeniously linking attributes of a spark event to valve degradation, the inventors herein have developed an inexpensive and reliable method to determine valve degradation. In this regard, the method may also help reduce engine emissions if valve degradation occurs. Further, the method does not require pressure transducers or other function specific transducers; it simply uses an existing spark plug to make a determination of valve degradation. As such, the system and method may be implemented at a low cost.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.